High speed automotive diesel engines capable of 4500 to 5000 r.p.m. that have been in mass production are the Daimler-Benz engine or variations of the Ricardo "Comet" design or the Daimler-Benz diesel engine. The engines have all been 2 valves; OHV or OHC design. Diesel engines have their own distinctive complications due to the high compression ratios needed to run these engines. Firstly, a strong bridge is desired in the cylinder head between the exhaust and intake valves to allow for sufficient strength and for providing a coolant passage.
Secondly, valve lift during valve overlap is reduced due to the small clearance between the valves and the piston at top dead center (TDC) to prevent hitting of the valve into the piston. In theory, the maximum valve lift is in direct proportion to the inner seat diameter of the valve. Further valve lift beyond the calculated maximum valve lift provides for very little additional air flow due to the restriction of the diameter at the throat of the port. The valves on these prior art diesels are made substantially smaller than valves for an equivalent version of a gasoline spark ignition engine to allow for larger valve bridges to withstand the higher diesel firing 1 pressures. The smaller size and reduced lift of the valves gravely affect the volumetric efficiency and air flow of diesel engines, especially prior-art engines with only two valves.
With two-valve engine technology, larger valves and relatively higher valve lift characteristics are used as compared to four-valve engine technology. To achieve relatively higher lift characteristics, proper valve dynamics dictates relatively long periods to open and close the valves. Mechanically, the valve spring are choosen to exert relatively great forces on the valve train components. This increases valve-train friction and causes a power loss. In a typical two-valve pushrod engine, the dynamic weight of the valve train mechanism (cam tappet, pushrod, rocker arm, valve, spring, retainer, and keepers) will be about four times higher per valve than that of the smaller valves of the proposed four-valve design. The proposed design has much smaller and lighter tappets, springs, retainers, and keepers as well as the elimination of rocker arms and pushrods. It is expected that the total valve train friction of the four valve design will be less than half that of the two valve, push rod design.
The valve lift is adversely affected at the critical valve overlap period when the intake valve is beginning to open and the exhaust valve is closing. The limitation of valve lift at this time affects the thorough flushing of the exhaust gases and inhibits the cylinder filling process for the subsequent cycle. The reduced valve lift during the overlap period, and the large valve periods force a late intake closing and an early exhaust opening. A late intake closing reduces the effective compression ratio with detrimental starting and running consequences, and greatly reduces the volumetric efficiency at low speeds. An early exhaust valve opening wastes energy and raises the exhaust gas and exhaust valve temperature which forces the use of more expensive and exotic high temperature valve and seat materials. An early exhaust closing raises the probability of a recompression spike, or "lock-up" at TDC during the scavenging or overlap portion of the cycle at high speed and high load, when in some engines, there is not sufficient real time available for a complete evacuation of the exhaust gases. Recompression spikes, apart from inhibiting the proper gas-flow process and reducing power and volumetric efficiency, consume energy by creating negative work during the exhaust stroke near TDC. The exhaust valve closing must occur late enough during an extended overlap period with the intake valve to prevent a recompression spike near top dead center.
Diesel engines have been able to tolerate these problems at low speeds. The operation at low speed provides sufficient time for the air flow through the intake and exhaust valves to pass into and out of the cylinder even with a delayed intake valve opening or early exhaust valve closing. However, the problems associated with valve timing and air flow lag become magnified at high speeds. The late intake valve opening becomes detrimental and substantially decreases the volumetric efficiency and compression chamber pressure and temperature. The combination of a late intake opening and an early exhaust closing provides for increased risk of a recompression spike at high speed operation. However the high compression ratios of a conventional high-speed I.D.I. diesel engine with the piston at top dead center being very close to the valves dictate that the intake valves cannot be opened early due to crashing into the piston and the exhaust valve cannot be closed late due to the crashing of the piston into the exhaust valve. The unnatural valve timings detract from the potential high-speed capability of this type of diesel engine.
A major compromise of these prior-art high-speed, 2-valve engines results when the valve opening duration must be extended at high speed to improve the volumetric efficiency lost by the lack of proper overlap. In every case, the intake valve closes excessively late in the compression stroke, and the effective compression ratio, effective compression pressure and effective compression temperature are too low even for the high speeds. When such engines run at low speeds, the same applies, but in addition, the volumetric efficiency suffers because the upward piston motion on the compression stroke "spit-back" into the intake manifold the air which has already been admitted into the engine and for which energy has been spent. Negative work (more energy wasted) also results from returning certain amounts of this already-admitted air back into the intake manifold. The situation is further aggravated at cranking speeds, especially cold when the batteries are weak and the oil is thick and said speeds are in the order of 100-150 rpm. The effective compression ratio under said conditions is lowered so much that cold startability is greatly affected or impossible.
Another compromise is when engines attempt to use the same components for both gasoline and diesel engines. In the past, gasoline and diesel engines did not have many components that were interchangeable. Neither engine could be economically and feasibly converted to the other type. Because of the diesel's lower sales volume, the diesel engine becomes more expensive and less desirable.
Lower speed diesel engines having 2600-2800 r.p.m. have been developed with four valves to circumvent the breathing and valve timing problems of their prior art two-valve counterparts. The use of four valves decreases the restriction of air flow through the valve openings. It is common to have a 40% increase in total air flow area as compared to a conventional two-valve engine. Since each valve is smaller compared to a two-valve per cylinder engine, the maximum valve lift can be reduced. The valve stem can also be made smaller in diameter, shorter and with less mass. Because each valve is lighter, the springs can be made softer. The softer springs provide for a more efficient engine. Because there is decreased lift for each valve, the cams on the cam shaft can be contoured, if desired, for less valve duration with reduced valve dynamic problems. Furthermore, the timing in which the valve needs to be open can be shortened because there is less lift to contend with even with the same valve accelerations. A further improvement results from lower exhaust valve temperatures, since each smaller exhaust valve has less ratio of surface area for heat pick-up to seat area for heat rejection. The result is the valves runs much cooler. Furthermore, with the larger valve opening areas but overall smaller diameter valves, the designer can afford to make slightly wider bridges and such bridges are shorter in length, which, added to the lower valve mass and running temperature, results in much lower valve head deflections and longer valve and seat life. Additionally, with shorter valve periods the exhaust valve spends less time open, exposed to the exhaust gases, and more time closed and rejecting heat, which lowers the valve temperature even further.
The few known four-valve diesel engines have a centrally located pre-combustion chamber with a large-diameter transfer passage between the pre-combustion chamber and main combustion chamber. The burn or combustion duration was short enough for these lower speed engines but are not short enough to be adapted to high-speed diesel engines. Furthermore, the pistons in these diesel engines tended to have heat checking and cracking due to the torch-like flame exiting the pre-combustion chamber and impinging at a near perpendicular angle onto the piston surface. These engines suffered from two extra combustion drawbacks: first, by the disposition of their valve train, with pushrods, rocker arms and rocker-arm bridges operating the valves, the two intakes (and the two exhausts) were in an axis transversal to the main axis of the engine and their porting and flow characteristics were not what would be considered appropriate today. With such disposition of the valves, swirl, or rotational air movement about the cylinder axis, was generated and, although swirl may have been beneficial by the general philosophy of the combustion chamber, it was a weak swirl and extracted a high air-flow penalty to generate the swirl. Secondly, the straight transfer passage of the pre-cup did not promote swirled air motion into the pre-cup. The results were increased combustion knock, NOx, hydrocarbons, smoke and particulate than a swirling pre-cup could have provided. Yet, engines from both the Caterpillar and Teledyne Continental Motors were first certified for low emissions by meeting the 1975 truck emission standards, indicating the great improvements to be obtained with more modern technology.
Modifications to pistons have also increased efficiency of engines. Many engines have a piston with a recess to form part of the combustion chamber or to enhance air swirl. The "Comet" diesel engine had a "spectacle-shaped" recess in its piston to form the main combustion chamber. It was not aligned or coordinated with the valves to act as a pocket to increase the clearance between the valves and the piston at TDC. Air tumble, another means of providing in-cylinder air motion (rotating air motion about an axis parallel to the engine centerline), has only been used in gasoline spark ignited engines. What is needed is a high-speed diesel engine with highly improved air flow and power output with lower fuel consumption, smoke, particulates and gaseous emissions, improved startability and reduced combustion noise and harshness, and offering increased durability of valves and piston. Means must be provided to achieve such objectives, such as a direct-acting double overhead camshaft configuration to achieve a lightweight, low stress, low power consumption, stiff and quiet valve train with superior air flow characteristics, and a combustion system capable of accomplishing fast and clean combustion both in the early (pre-combustion, in the pre-combustion chamber) and late (main chamber) combustion. The subject of this patent provides all the elements to achieve such results in a modern, relatively inexpensive engine package, with a possible cylinder head even converted from appropriate gasoline engine counterparts featuring the airflow capacities of a four-valve design with narrow valve-included angle and DOHC, preferably a mechanically stiff direct-acting design with shorter valve periods, increased effective compression ratio and decreased nominal compression ratio, with a highly turbulent mixture generated in the pre-chamber for quick ignition and fast burn, followed by combustion in the main chamber being characterized as a fast process based on a combination of air tumble, squish, mixing with the fuel and products of combustion exiting from the pre-chamber and burning quickly and efficiently by maintaining said pre-chamber products airborne while mixing and burning with the air in the main chamber. The process takes advantage of, and is based on, appropriate recesses incorporated in the piston which, apart from functioning as mini-combustion chambers, also provide for valve pockets to allow for the proper valve events and lifts without combustion or manufacturing compromises, and which contribute to an even thermal loading. The combination of volumetric efficiencies and valve timings providing previously unheard of startability, smooth, quieter combustion and reduced firing pressures, even while producing increased power outputs and lower gaseous and visible emissions.